Pasting via dialog should provide accessible dataTransfer

[Tomasz Jakut]

Currently pasting rich text into dialog could strip all additional information (e.g. styling) and pass only plain text into the editor. It's especially visible when pasting from Word:

1. Open attached Word document.
2. Select part of the sentence (e.g. first two words).
3. Click "Paste" button to open paste dialog.
4. Paste into dialog.

The text pasted into dialog is converted into plain text and in this form is then pasted into the editor.

There should be a way to get the original, raw pasted data from dialog, preferably from native dataTransfer. However, due to limitations of clipboard API, it can be fetched only synchronously. Probably the best way to get it from editor's dialog is to create new event, which will be fired after pasting into dialog.

The way to obtain raw data should be universal, as in the future editor may need to get some other data type, not only text/html. In this case creating a snapshot of native dataTransfer and passing it to the event should be a sufficient solution.

[Marek Lewandowski]

Fixed by #16954.